Electron impact time of flight spectrometer

ABSTRACT

An electron impact time of flight spectrometer including means providing monochromatic electrons of a predetermined kinetic energy which are caused to impinge upon a sample material so that some of the electrons experience an energy transition in passing through the material. The electrons are then differentially accelerated such that electrons having similar energy levels will be caused to converge and arrive at a detector in tightly formed bunches.

United States Patent [72] inventors Weston A. Anderson Palo Alto; John C. llelmer, Menlo Park, both oi, Calif. [2|] Appl. No. 734,690 22] Filed June 5, 1968 [45] Patented June 1,1971 [73] Assignee Val-inn Associates Palo Alto, Calif.

[54] ELECTRON IMPACT TIME OF FLIGHT SPECTROMETER 10 Claims, 5 Drawing Figs. [52] US. Cl. 250/49.5, 250/419, 328/233 511 Int. Cl H01] 37/00 [50] Field oiSearch 250/419 G, 41.9 (1),49.5 (0), 49.5 (1); 328/233 [56] References Cited UNITED STATES PATENTS 2,429,558 10/1947 Marton 250/495 ABSTRACT: An electron impact time of flight spectrometer including means providing monochromatic electrons of a predetermined kinetic energy which are caused to impinge upon a sample material "so that some of the electrons experience an energy transition in passing through the material. The electrons are then differentially accelerated such that electrons having similar energy levels will be caused to converge and arrive at a detector in tightly formed bunches.

DELAY RECORDER ELECTRON IMPACT TIME OF FLIGHT SPECTROMETER STATEMENT OF THE INVENTION The invention relates in general to electron impact spectroscopy and more particularly to a novel method and apparatus for determining the excited states of atoms and molecules by measurement of the energy loss experienced by a quantity of low-voltage electrons in passing through a sample material.

PRIOR ART It has long been known that by passing an electron beam through a group of molecules and then measuring the energy loss experienced by the electrons due to molecular energy transitions certain characteristics of the molecular structure can be determined. Since some of the electrons will lose discrete amounts of energy corresponding to the molecular energy transitions, these transitions can be detected by measuring the energy spectrum of the resulting electron beam. However, the energy transfer between the incident electrons and the unknown molecules is dependent on the initial energy of the electrons, consequentlythe resolution of any detected information depends on the degree of homogeneity of the incident electrons. Ideally, a monochromatic source could provide a source of monoenergetic electrons which would permit a high resolution energy spectrum to be detected, but since no practical monochromatic electron source is currently available, the source of electrons is typically a thermionic emitter which characteristically has an energy spread of approximately onehalf volt among its emitted electrons. For example, the energy half-width of a beam of electrons from a good thermionic emitter can be determined according to the equation (1) AE=2.54T/11600 where Tis the temperature of the cathode in K., and

AE is the full-width at half maximum expressed in electron volts (e.v.). Even with low work function oxide cathodes, operation at temperatures low enough that A is less than 0.25 e.v. is not practical, hence the approximation of one-half volt mentioned above.

In order to achieve a reasonable signal resolution prior methods have generally included means for selecting certain monoenergetic (monochromatic) electrons and rejecting others. Obviously, by so doing the sensitivity of the apparatus is substantially reduced. In one prior art apparatus including means for reducing the thermal energy spread the initial electron beam is passed through a magnetic or electrostatic energy analyzer which selects a group of electrons with less energy spread. This, however, is done at the expense of beam intensity. Simpson, Review of Scientific Instruments, Vol. 35, I698 (I964), has developed an apparatus of this type with which he has achieved an energy spread of 0.005 volts. But the inherent complexities of Simpsons deviceand low sensitivity which accompany the use of his technique have made it desirable to search for other methods and apparatus which do not require energy selection and are thus independent of the initial energy distribution of the electron beam.

OBJECTS OF THE INVENTION It is therefore a principal object of the present invention to provide an electron-impact spectrometer the signal resolution of which is independent of the initial energy spread among the electrons at their source.

Another object of the present invention is to provide a high resolution electron impact spectrometer which is capable of utilizing substantially all of the thermionically emitted electrons admitted into the analyzer notwithstanding their nonhomogeneous energy distribution.

Still another object of the present invention is to provide a time-of-flight electron impact spectrometer using a thermionic electron source and having means for compensating for nonhomogeneous energy conditions among the electrons admitted into said analyzer.

Still another object of the present invention is to provide a means for monochromatizing electrons which is suitable for use in electron impact spectrometers and the like.

Still other objects and advantages of the present invention will become apparent after a reading of the following description of preferred embodiments illustrated in the drawing wherein:

IN THE DRAWINGS FIG. 1 is a schematic representation of an electron rnonochromator for use in accordance with the present invention. FIG. 2 is a schematic representation of an electron impact spectrometer in accordance with the present invention. FIG. 3 is an electron pulse-time spectrum illustrating the operation of the present invention. FIG. 4 is an illustration of an electron-impact energy spectra obtainable using the present invention. FIG. 5 is a schematic representation of an alternate embodiment of an electron impact spectrometer in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with preferred embodiments of the invention novel time-of-flight spectrometer apparatus is described including means providing monochromatic electrons of a predetermined kinetic energy level for impingement upon a given analytical sample of material.

An electron rnonochromator suitable for use in spectrometer apparatus of the type to be described hereinafter is sche matically illustrated in FIG. 1 of the drawing. A thermionic electron source such as the filament 10 is located at one end of an evacuable envelope 14 which is evacuated by a suitable pumping means 16. Disposed proximate the filament 10 is a grid 18 which is periodically gated positive for short periods of time, i.e., of'lO' seconds duration, by pulses applied to the terminal 20. Short bursts of electrons are thus caused to be propelled through the grids l8 and 22 and into the field free drift tube 24 wherein the initially tightly grouped bunches of electrons, while traveling down the tube 24, tend to debunch and spread out in the axial direction due to the different energy characteristics of the respective electrons. The energy distribution among the electrons, as mentioned previously, is the result of the electron emission characteristics of the thermionic emitter 10.

In order to monochromatize the electrons which reach the far end of the drift space a ramp voltage is applied directly to the drift tube 24 so that a time varying electric field will be created between the end 25 of the drift tube 24 and a grounded electrode 26. This voltage is generated by a ramp generator 28 which is synchronized with the pulses applied to gating grid 18 so that the faster electrons will be decelerated, the slow electrons will be accelerated and the electrons of average velocitywill pass unaffected. Thus, the stream of electrons which pass through grid 26 into the target area all have the same kinetic energy and are in fact monochromatized (raised to the same energy level) even though spatially separated.

The voltage V,(t) which must be generated by ramp generator 28 and applied to drift tube 24 so as to create the differentially accelerating field between the tube end 25 and the grid 26 may be expressed as is the time required for an electron of kinetic energy e.v. to traverse the drift tube 24 of length L.

Referring now to FIG. 2 of the drawing there is shown a preferred embodiment of an electron impact spectrometer in accordance with the subject invention and including an alter native modification of the electron monochromator described above. The monochromator is disposed at one end of the envelope 30 and includes a filament 34 for generating electrons, a gating grid 36 normally biased heavily negative, a drift tube 38 and a field control grid 42.

Although most of the generated electrons are prevented from entering the analyzer by the grid 36 biased heavily negative, short bursts of electrons are periodically caused to pass into the analyzer and drift axially down the drift tube 38 maintained at ground potential. These bursts are caused by gating pulses of about seconds duration applied to grid 36 via terminal 40 by external pulse generating means. Each burst allowed to enter drift tube 38 has an inherent energy spread among the respective electrons of about one-half volt due to the emission characteristics of the thermionic source as mentioned above. Consequently, the velocities imparted to the respective electrons vary accordingly such that by the time they reach the end of the drift tube 38 they have spread out along their path and are no longer tightly bunched together.

In order to monochromatize the electrons emerging from the end 41 of drift tube 38 a dynamic electric field is created between the end 41 of the drift tube 38 and the electrode 42 in a similar manner as is disclosed above with reference to FIG. 1. Whereas in the FIG. 1 embodiment the field changing potential was applied to the drift tube 24, the potential in this embodiment is applied to the grid 42 thus creating the appropriate dynamic electric field region between the end 41 of drift tube 38 and the grid 42.

It should be noted, however, that the two methods are readily interchangeable since both monochromatize the emerging electrons using substantially the same electrode structures and the only material difference lies in the manner in which the voltage V,(t) for creating the monochromatizing field between the end of the drift tube 38 and the grid 42 is applied.

In practice the source electrons have only a limited spread in their relative velocities and the time which is required for each group of electrons to pass through the gap between the end 41 of drift tube 38 and grid 42 is short compared with the pulse repetition rate. Where this is the case, the gap voltage may be provided by the linear portion of the waveform of a sinusoidal oscillator instead of using the ramp generator shown in the FIG. 1 embodiment. Since the crossover portion of the sinusoidal wave nearly approximates a straight line, it will be apparent that a dynamic electric field which changes from negative to positive with respect to the direction of travel of the electrons can be created between drift tube 38 and grid 42 using the sinusoidal source. Thus, by selecting a sinusoid of a suitable frequency phase and amplitude for application to grid 42 the electric field created will cause the faster electrons to be decelerated and the lagging electrons to be accelerated such that all of the electrons emerging from the field will have like energy potentials.

The sinusoidal voltage supply means 44 is synchronized with the input gating pulses applied at terminal 40 by a suitable delay means 46 which is timed in accordance with equation (2) and is connected to grid 42 through a centertap of the coupling network 48 such that the positive going zero crossover portion of the sinusoidal wave occurs just as the electrons having average energy appear at the grid 42. Consequently, the field retards the fast electrons and accelerates the slower electrons so that as they pass through the grid 42 toward sample chamber 50 each electron has the same kinetic energy.

The gas cell 50 having entrance and exit slits 52 and 54 respectively, is provided in the electron path so that the electrons transiting through the apparatus will pass through a sample gas introduced into the cell 50. In order that the monochromatized low energy electrons be caused to enter the cell 50 at an optimum velocity a suitable accelerating potential is applied to the housing of cell 50 by a voltage source 56 so as to create an accelerating field between it and grid 42. As

the respective monoenergetic electrons pass through the sample gas some of them will experience discrete losses of energy as they pass in close proximity to the gas molecules due to the molecular energy transitions which occur. These energy losses have the effect of decreasing the velocity and hence the kinetic energy of the affected electrons by an amount proportional to the amount of energy transferred to the gas molecules thus enabling an energy analysis of the gas to be performed by measuring the energy losses sustained by the electrons.

Where it is desirable to decelerate the electrons emerging from the cell 50 back to the low energy level with which they left the monochromator, the same voltage applied to grid 42 is applied to grid 57 to create a decelerating field between cell 50 and grid 57. The use of the same voltage on both of grids 42 and 57 is possible since in practice the actual spatial separation between the grids 42 and 57 is small enough to be disregarded. However, in those apparatus wherein it is required that the sample cell 50 be so large as to require a significant electron transit time therethrougn, it will be necessary to include appropriate means for modifying the signal applied to the grid 57.

In order to cause those electrons having common energy levels to be rebunched at the end of drift tube 60 a bunching voltage is applied to grid 58. This voltage is also obtained from the sinusoidal source 44, but the portion of the wave selected is a positive going portion chosen such that the zero crossover is synchronized with the arrival of the electrons which have suffered no energy loss in passing through the cell 50. Thus, as the spatially distributed stream of electrons of varying energies enter the field between grids 57 and 58 the lagging electrons receive greater acceleration than the first arriving electrons such that those having similar potentials will tend to regroup as they travel through the second drift tube 60. The voltage at grid 58 required to rebunch the electrons at the end ofdrift tube 60 is given by where 1' is the time required for an electron of kinetic energy e.v. to traverse the second drift tube 60. This equation is appropriate where the travel time through the sample cell 50 is negligible.

By applying the voltage V (t) all of the electrons having similar energy levels e.v. will be caused to converge and arrive at the end of drift tube 60 at the same time. Ifone were able to view the stream of electrons passing out of the end of the tube 60 when nitrogen gas was the sample material being analyzed he would see an electron pulse-time spectrum of the type which is illustrated in FIG. 3. The large peak at the left corresponds to the first group of electrons to arrive at the detector. This group is comprised of those electrons which passed through the sample without suffering any appreciable energy loss. The later arriving peaks, at the right in FIG. 3, are representative of those groups of electrons which have experienced various degrees of energy loss while passing through the sample.

In FIG. 4 the equivalent energy spectrum of those peaks shown at the right in FIG. 3 is illustrated. From this energy distribution spectrum, which is referenced back to the energy level of the electrons which suffered no energy loss during their passage through the sample, an experienced investigator can extrapolate much useful data regarding the characteristics of the sample material.

In the present embodiment the electron energy distribution is viewed directly by gating a grid 62 in delayed synchronism with the input gating pulses supplied at 40. In so doing the electrons arriving at the end of the tube during the short interval which the gate is open are allowed to pass through to a collector electrode 64 where they produce a current flow in line 66 which can be measured by a meter 68 and recorded on a recorder 70 responsive to the meter 68. Using the apparatus thus far described only one small segment of the impact spectrum can be detected. But, by applying a variable potential from a source 72 to the drift tube 60 it is possible to create a variable accelerating field between the end 61 of drift tube 60 and gating grid 62 so that the stream of electrons may be selectively accelerated causing any desired portion of the electron stream to arrive coincident with the opening of the gate 62 and thus be indicated on meter 68.

The variable source 72 thus provides a means by which any desired portion of the energy spectrum may be swept, or scanned, so as to produce a graphical energy spectrum such as that shown in FIG. 4. in FIG. 4 the current I measured on meter 68 is shown as the voltage of source 72 is swept to produce an energy loss spectrum of N in the range of 8 to 12.5 volts. The spectrum is referenced to the detected peak at zero volts (not shown) which represents those electrons which passed through the sample without experiencing any energy loss.

Referring now to FIG. 5 a simplified alternative embodiment of the apparatus shown in FIG. 2 is disclosed. In this embodiment the voltage supplied at grid 42 does not necessarily provide monochromatized electrons for passage through the sample. it provides a bunching voltage which causes the electrons to regroup at the end 61 of drift tube 60 in accordance with the different energy levels as described above. In this modification the bunching voltage V 0) is generated by source 44 and applied through coupling network 48 to grid 42 while grid 58 is eliminated. Otherwise, the operation of this embodiment is similar to that of HO. 2. This simplified embodiment is suitable and provides acceptable spectra with good resolution for sample materials where the relative energy of the electrons impinging on the sample is not highly critical.

It will, of course, be apparent to those of skill in the art that other modifications of the present invention can be made. For example, other means of sweeping the spectra may be used. One readily apparent example would be to provide a variable delay means between the pulse input 40 and the gating grid 62 so that the gate could be opened at different electron transit times so as to produce the same type of spectral output. Likewise, the form of monochromator disclosed with reference to FIG. 1 could be substituted for that shown in FIG. 2. The device could also be modified to utilize'ions as the charges particles in place of the electrons which are caused to pass through the sample gas.

After having read the above disclosure, many more alterations and modifications of the invention will be apparent to those of skill in the art and it is to be understood that this description of preferred embodiments is for purposes of illustration and is in no manner intended to be limiting in any way.

Accordingly, we intend that the appended claims be interpreted as covering all modifications which fall within the true spirit and scope of our invention.

What we claim is:

1. An electron monochromator suitable for use in an electron spectrometer comprising:

a source of electrons having an initial relative energy spread,

drift tube means for providing a field free drift space through which said electrons may be passed,

electronic gating means disposed between said source and one end of said drift tube means for periodically selecting discrete bunches of said electrons and causing them to pass through said drift tube means, and

means creating a dynamic electric field at the other end of said drift tube means for differentially accelerating said electrons passing out of said drift tube means to produce bunches of monoenergic electrons.

2. An electron monochromator as defined in claim 1 wherein said differential accelerating means comprises electrode means disposed proximate said other end of said drift tube means and a voltage supply means synchronized with said gating means for providing a predetermined time varying voltage to said electrodes for creating said dynamic electric field and causing the energy potential of all of said electrons to be the same.

3. An electron analyzer comprising;

a source of electrons having an initial relative energy spread,

first means providing a first field free drift space through which said electrons may be caused to pass,

gating means disposed between said source of electrons and one extremity of said drift space for selecting discrete bunches of said electrons emitted from said source and causing them to pass through said drift space,

second means providing a second field free drift space through which said electrons may be passed,

means creating a dynamic electric field between said first and said second means for differentially accelerating said electrons passing out of said first drift space to produce bunches of monoenergic electrons which are caused to converge and form discrete bunches having different energy levels spaced in time relative to each other upon reaching another extremity of said second drift space,

a sample chamber between said first and said second means lows electrons to pass therethrough to a collector means only during short intervals of time, and variable electron accelerating means for enabling the selective determination, according to energy level, of which bunch of electrons is to be allowed to pass through said gate during said short intervals of time.

S. An electron analyzer as described in claim 3 wherein said electron accelerating means comprises an electrode means for creating a time varying electric field between the adjacent extremities of said first and second drift space means in response to a time varying voltage applied thereto and voltage source means synchronized with said gating means for applying a predetermined time varying voltage to said electrode.

6. An electron analyzer as described in claim 5 wherein said voltage source means includes a sinusoidal oscillator means having a sinusoidal output of such frequency, phase and amplitude that when applied to said electrode means it causes said time varying electric field to change linearly during the time of transit of said electrons through said field.

7. An electron impact spectrometer apparatus comprising:

a source of electrons having an initial relative energy spread,

first drift tube means providing a first field free drift space through which said electrons may be caused to pass,

gating means disposed between said electron source and.

one end of said drift tube means for causing discrete bunches of said electrons to be propelled through said drift tube means,

a sample chamber through which said electrons may be passed so as to enable an energy transformation to occur between said electrons and a sample disposed in said chamber,

means creating a dynamic electric field at the other end of said drift tube means for differentially accelerating said electrons passing out of said drift tube means to produce bunches of monoenergic electrons,

second drift tube means for providing a second field free drift space through which said electrons are passed after passing through said sample chamber, and

means for detecting said electrons as they emerge from said last mentioned drift tube means so as to provide an indication of the energy loss sustained by said electrons during their transit through said sample.

8. An electron impact spectrometer as recited in claim 7 wherein said detecting means includes acollector means disposed proximate the downstream end of said second drift tube means, electron gating means disposed between said collector means and said downstream end of said second drifttube means for allowing electrons to pass therethrough to said collector means only during short intervals of time, and means for creating a variable accelerating electric field between said downstream end of said second drift tube means and said electron gating means so that electrons of selected energy levels may be preferentially caused to pass through said gating means during said short periods of time.

9. An electron impact spectrometer as recited in claim 7 wherein said electron accelerating means comprises means for creating a time varying electric field between said other end of said first drift space means and said sample chamber means in response to a time varying voltage applied thereto and voltage 

1. An electron monochromator suitable for use in an electron spectrometer comprising: a source of electrons having an initial relative energy spread, drift tube means for providing a field free drift space through which said electrons may be passed, electronic gating means disposed between said source and one end of said drift tube means for periodically selecting discrete bunches of said electrons and causing them to pass through said drift tube means, and means creating a dynamic electric field at the other end of said drift tube means for differentially accelerating said electrons passing out of said drift tube means to produce bunches of monoenergic electrons.
 2. An electron monochromator as defined in claim 1 wherein said differential accelerating means comprises electrode means disposed proximate said other end of said drift tube means and a voltage supply means synchronized with said gating means for providing a predetermined time varying voltage to said electrodes for creating said dynamic electric field and causing the energy potential of all of said electrons to be the same.
 3. An electron analyzer comprising; a source of electrons having an initial relative energy spread, first means providing a first field free drift space through which said electrons may be caused to pass, gating means disposed between said source of electrons and one extremity of said drift space for selecting discrete bunches of said electrons emitted from said source and causing them to pass through said drift space, second means providing a second field free drift space through which said electrons may be passed, means creating a dynamic electric field between said first and said second means for differentially accelerating said electrons passing out of said first drift space to produce bunches of monoenergic electrons which are caused to converge and form discrete bunches having different energy levels spaced in time relative to each other upon reaching another extremity of said second drift space, a sample chamber between said first and said second means through which said electrons may be passed so as to enable an energy transformation to occur between said electrons and a sample disposed in said chamber, and means for detecting said bunches of electrons as they emerge from said drift space.
 4. An electron analyzer as recited in claim 3 wherein said detecting means includes an electron gating means which allows electrons to pass therethrough to a collector means only during short intervals of time, and variable electron accelerating means for enabling the selective determination, according to energy level, of which bunch of electrons is to be allowed to pass through said gate during said short intervals of time.
 5. An electron analyzer as described in claim 3 wherein said electron accelerating means comprises an electrode means for creating a time varying electric field between the adjacent extremities of said first and second drift space means in response to a time varying voltage applied thereto and voltage source means synchronized with said gating means for applying a predetermined time varying voltage to said electrode.
 6. An electron analyzer as described in claim 5 wherein said voltage source means includes a sinusoidal oscillator means having a sinusoidal output of such frequency, phase and amplitude that when applied to said electrode means it causes said time varying electric field to change linearly during the time of transit of said electrons through said field.
 7. An electron impact spectrometer apparatus comprising: a source of electrons having an initial relatiVe energy spread, first drift tube means providing a first field free drift space through which said electrons may be caused to pass, gating means disposed between said electron source and one end of said drift tube means for causing discrete bunches of said electrons to be propelled through said drift tube means, a sample chamber through which said electrons may be passed so as to enable an energy transformation to occur between said electrons and a sample disposed in said chamber, means creating a dynamic electric field at the other end of said drift tube means for differentially accelerating said electrons passing out of said drift tube means to produce bunches of monoenergic electrons, second drift tube means for providing a second field free drift space through which said electrons are passed after passing through said sample chamber, and means for detecting said electrons as they emerge from said last mentioned drift tube means so as to provide an indication of the energy loss sustained by said electrons during their transit through said sample.
 8. An electron impact spectrometer as recited in claim 7 wherein said detecting means includes a collector means disposed proximate the downstream end of said second drift tube means, electron gating means disposed between said collector means and said downstream end of said second drift tube means for allowing electrons to pass therethrough to said collector means only during short intervals of time, and means for creating a variable accelerating electric field between said downstream end of said second drift tube means and said electron gating means so that electrons of selected energy levels may be preferentially caused to pass through said gating means during said short periods of time.
 9. An electron impact spectrometer as recited in claim 7 wherein said electron accelerating means comprises means for creating a time varying electric field between said other end of said first drift space means and said sample chamber means in response to a time varying voltage applied thereto and voltage source means synchronized with said gating means for applying a predetermined time varying voltage to said electrode.
 10. An electron impact spectrometer as recited in claim 9 wherein said voltage source means includes a sinusoidal oscillating means having a sinusoidal output of such frequency, phase and amplitude that when applied to said electron accelerating means it causes said time varying electric field to change linearly during the time of transit of said electrons through said field. 